The present invention relates to a for removing and/or re-installing manhole lids in an apparatus under pressure.
Removing and re-installing manhole lids is particularly delicate owing to the radiation in the case of a nuclear apparatus such as a steam generator, and therefore the invention will be described for this typical application, it being understood that the invention is also applicable in each case when a mechanically similar problem arises.
In the particular case of a steam generator, the manhole is located in the lower part of the apparatus which is usually disposed vertically, i.e. in the lower hemispherical bottom. The lid is inclined at 40.degree. relative to the vertical axis of the steam generator and located on legs projecting from the hemispherical bottom. Such lids must be secured so that the high pressure inside the apparatus may be kept. Removing and re-installing such lids require the lids to be positioned and the securing bolts to be tightened.
Equipment making such operations easier exists at present. The most widely known equipment is mainly of two types.
(a) Hinged system
When being removed the lid is made integral, through bolts, with a supporting angle-iron perpendicular to the axis thereof. This angle-iron may be rotated about an axis perpendicular to its length and relative to a rotating point situated above the legs. Through a fastening lug welded on the hemispherical bottom supporting this axis, the latter may function as a hinge for pivoting the lid and clearing the opening. However, this technique is only valuable when the lid is fixedly screwed on the bottom.
(b) Hinged system with crank screws
When using studs with nuts, the angle-iron may be made integral with the lid only through guides and crank screws so that the lid may be shifted on the opening axis, while being released from the studs before pivoting about the above mentioned hinge, thereby allowing access to the manhole. As already described, the lid may be maintained on its legs either through screws or studs and nuts. According to the usual practice, these screws or nuts are tightened with torque wrenches by acting successively upon each screw or nut in several crosswise movements. The main drawbacks of such a system are as follows:
Several workmen are operating simultaneously on the same lid. PA1 Important manual labour. PA1 Substantial working time under the steam generator. PA1 As a direct result, a substantial exposure to radiation. PA1 Utilization of a supporting means at each manhole. PA1 (a) successive step by step systems, PA1 (b) simultaneous tightening systems. PA1 (1) the stud can be stressed below its yield strength, PA1 (2) the second nut can contact the lid when the stud is stretched, PA1 (3) the stress can be relieved within the stud, the required residual stress being provided by the second nut. PA1 (1) be disposed under the steam generator, PA1 (2) locate the ring on the lid, PA1 (3) screw and unscrew the nuts after tightening, all these operations and any other excepted, being performed by this assembly, it being understood that the lid itself should be always supported and located through a system such as described under (b) above. PA1 (a) it avoids the problems relating to the space occupied and the long and tedious manual operations; PA1 (b) it limits the number of operators under the steam generator (one workman many be sufficient), PA1 (c) it substantially reduces the time devoted to the above described operations,
As every steam generator comprises several manholes, the supporting means must accordingly be provided in the same number as the manholes.
The presently used techniques for fastening lids on the legs of the hemispherical bottoms of steam generators are principally of two types:
The purpose is either to locate screws the heads of which are fastening the lid and the threaded portions of which passing through the bores of the lid are screwed in the bulge of the legs, or to locate studs a portion of which is also screwed in the legs, but which are fastening the lid through the intermediary of a nut also screwed on said studs. In order to maintain the pressure, it is necessary not only to provide the contact between the lid, the seal and the bottom legs, but also to provide a residual stress in the bolt system.
As already mentioned above, according to the usual practice, the screws or nuts are tightened with a torque wrench or spanner by successively acting upon each screw or nut in several crosswise movements, thereby uniformly distributing the stresses. Unfortunately, when the screws or nuts are so tightened, it is not possible to accurately define the residual stress within the bolt system.
In order to avoid such drawback, special tools are available for tightening the studs, thereby precisely determining the residual stress without the bolt system.
The stud is tightened through a small annular piston under a hydraulic pressure and pulling the stud.
For that purpose, use is made of a stud having three threaded portions, namely a first portion integral with the bottom of the steam generator, a second portion integral with the fastening nut of the lid and a third portion integral with a nut through which:
In order to uniformly distribute the stresses and with a view to saving time during the screwing and unscrewing operations, machines for simultaneously tightening all the studs are available in the industry. Said machines comprise principally a ring with bores through which are passing the threaded heads of the studs and pistons bearing on the lid for tightening the studs after having located the third nut connecting the studs with the machine.
The systems of the first type do not require any special supporting tools and they may be manually handled, whereas the systems of the second type require a special supporting means owing to the space occupied and the weight thereof, namely from about 250 to 300 kg to which one should add the pump assembly for raising the hydraulic-pneumatic pressure and weighing about 50 kg.
Heretofore, special tools only intended to support a system for simultaneously tightening studs, are known. A first known system comprises a modular wheeled carriage provided with linked arms supporting the ring provided for the simultaneous tightening so that this assembly can:
In order to carry out the three foregoing operations, a smooth floor allowing the carriage to roll must be provided, which is not always the case.
Another system provides a mechanization of the operations described for the hinge and crank system, said system permitting to support both a ring for simultaneously tightening the studs and the lid itself. Of course, such a system is a substantial improvement, but it requires the location of an assembly such as above described under each lid of the steam generator, i.e. two complete devices for a single steam generator, which is uneconomic.